On the range of validity of the semirelativistic magnetohydrodynamic equations

Plasmas with moderate flow velocity and sound speed, but large Alfvén speed have been described by the semirelativistic magnetohydrodynamics (MHD) equations. While these are correct when restricted to their range of validity, they may have the undesirable effect of predicting unphysical accelerations, much faster than the ones of classical MHD. We present a family of planar models on which the Lorentz force acts more forcefully in the semirelativistic approach, yielding a flow velocity which rapidly exceeds the limits within which the equations are valid.